40-50? You shouldn't require a server yet. I think you'll be fine with a good shared hosting provider. If you really want a server then I would go and get a P4. It should be more than enough and if you want to grow I think the P4 can support that.
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For your average vBulletin or phpBB forum, 40-50 active concurrent users is going to put you on the highest edge of what your average shared host can go.. but below the point where you should really need a full dedicated server.
Are you planning to grow quickly, or is this more of a niche forum that will sustain at 40-50 users on at a time? If you're planning to grow, maybe you should go with a low-end to middle-ground dedicated solution ($100 - 200/month.. not $350/month).. if not, a VPS in the $50-75/month range is definitely more cost-effective and shouldn't have a problem handling that forum.